Have you seen what happened in the finance world with respect to Russia recently? Feels to me like it is perfectly possible with the right Zeitgeist. Pretty much all the on/off ramps for Russian money are gone at the moment.

Which I acknowledge above. You can ban exchange to fiat but it will only reduce value, not completely eliminate it as there are protocols which derive value from defi ecosystem itself.

I.e people will start directly using these currency for payments (which they already do) without exchanging to fiat. The stability is maintained by the defi collateral and shared acceptance.

So you would have to ban everyone to accept crypto currency and so on as payment but that is very catastrophic decision with large implications.

It also doesn't help crypto completely eliminates centralized authorities when moving across borders.

> people will start directly using these currency for payments (which they already do) without exchanging to fiat

Without fiat exchange, People can only accept crypto in an economically feasible manner as payments up to the rate that they have expenses/outflows that can be paid in crypto. The price will only be able to stabilize at a price that is orders of magnitude lower than today. How much lower exactly depends on the health of the black market for exchanging with fiat.

So let's assume crypto is banned? Why is FIAT much better? What about another 2008? What about Quantitative Easing?

It's not solving much.

Because it's not speculative garbage like literally all crypto is.

Russia is a bit more complicated..half their economy is oil. The chance of import bans and Russia nationalizing oil companies is partly priced in.